I don't know, I think Anders is more of a skinny, weasel-shaped creeper than a huge one, Aveline's really the only huge party member.

But yeah, I don't think he takes Isabela very seriously as competition. Actually I don't think he takes her very seriously in general. I get the sense from that one bugged banter about the Tome that their relationship was meant to be a bit more dynamic, and whether they got along would depend on whether you Rivaled or Friended her, but as-is it seems like a simple case of him sometimes liking her, but never really respecting her.
(Looking back, I'm pretty sure the thing that sealed Anders' fate with me was the first time I heard their pre-Qun Crisis banter about the Tome; they're having a perfectly friendly conversation and then bam, "no, your hand isn't what I hear you've been showing," completely unrelated blatantly hypocritical s!ut-shaming out of absolutely nowhere. Can't blame that one on Justice.)
Speaking of Rivallng, a question for you Rivalmancers! I largely prefer the Friendmance, for multiple reasons, one of which is that it simply seems more coherent. I can tell what's going on in a Friendmance: in a nutshell, they have fun together. Isabela likes Hawke because Hawke takes her fun places with fun fights and fun treasure, and is a good person to get drunk with. Also, Hawke is a good person who helps people and looks out for her friends, which, per Merrill, Izzy seems to like, provided nobody's asking
her to be a good person, which Hawke doesn't really.
I can't figure out what the dynamic is in a Rivalmance. Her midstage QB and ambient dialog indicate she thinks Hawke is a boring, goody-two-shoes stick in the mud. But if Hawke is a boring, goody-two-shoes, Izzy's usually going to end up Friended, because she likes when you're nice to her, and forgiving and compassionate about her screw-ups rather than judgemental. And "boring" and "sexy" don't really go together. Meanwhile, the fastest path to Rivalry gains is to be a red Hawke and just be a total jerk to her. So which is it? Is Hawke boring, or a passionate jerk?
The closest I can figure is to picture it as something like her relationship with Aveline. But in that case why does she stick around? It's not for the sex, she follows Hawke around for three years before that happens. It doesn't seem like it'd be for the adventure, because the adventure would always be for the benefit of The Man - I don't see Izzy sitting around going "I don't want to leave Kirkwall just yet, I might get to rescue a guard from an ambush for no coin or return tonight!" It's not for love, that takes her years and her response to love is to run, anyway. Is she just cultivating Hawke's friendship because Hawke's badassery might be useful against Castillon?
And what should Rivalry sex have looked like? Because the scene we got is obviously Friendmance.